Long-term goals of the proposed work are to understand controlling mechanisms involved in the normal functioning of the adult mammalian hypothalamus. Specifically, this work centers on the magnocellular neuroendocrine cells (MNCS) of the hpothalamo-neurohypophysial system (HNS), consisting chiefly of the supraoptic (SON) and paraventricular (PVN) nuclei and their axonal projections. MNCs synthesize either oxytocin or vasopressin and, in response to physiological stimuli, release these peptides from axon terminals in the pituitary neural lobe or within the CNS. Well known to play important roles in water regulation, control of blood pressure, reproductive physiology and care of the young, this model system has yielded many insights into CNS functioning. So well studied are the peripheral effects of HNS outputs that even results from in vitro experiments with the SON and PVN can often be related directly to the functioning of the intact animal. In order to further the central mission of this work, this proposal focuses on two main aspects of HNS functioning: interactions among the MNCs themselves, particularly interneuronal coupling, and extrinsic and intrinsic factors that modulate MNC excitability. The proposed studies will use electrophysiological methods, intracellular an patch-clamp recording techniques in brain slices, accompanied by immunocytochemical and biochemical approaches, all aimed at uncovering fundamental mechanisms of CNS functioning. Specific aims are: Aim 1. To continue our investigations of interneuronal coupling in the SON. Experiments will further define the physiological conditions that enhance or diminish coupling, define the relationships between receptor activation and coupling, and determine cellular mechanisms that modulate MNC coupling. Aim 2. To continue investigation determining the mechanisms by which physiological changes alter MNC excitability. Specific studies will investigate the contributions of neuronal histamine, oxytocin, and intrinsic ionic mechanisms to MNC excitability and, thus, to mechanisms ultimately governing peptide release. Factors to be investigated include: depolarizing after potentials, plateau potentials, receptor-mediated conductances, and termination of bursts of phasic firing, as well as physiological state of the animal.